DC voltage converters have been widely used in kinds of industrial products and consumer products. Generally, the DC voltage converter is used to convert a high input voltage into a low output voltage which meets the requirements of circuit designs. There are three kinds of feedback control circuit designs of the DC voltage converter, which are the voltage mode, the current mode and the constant on time mode, respectively.
Conventionally, the circuit design of the constant on time mode is often used. Compared with the circuit designs of the voltage mode and the current mode, the switching efficiency of the upper-side switch and the lower-side switch is not determined by a clock generator in the circuit design of the constant on time mode. Instead, an on-time generator is used to generate a control signal, and the system end will accordingly adjust the switching efficiency of the upper-side switch and the lower-side switch.
FIG. 1 is a circuit diagram of a switching DC voltage converter. FIG. 2A, FIG. 2B and FIG. 2C are waveform diagrams showing the operation of the switching DC voltage converter in FIG. 1.
As shown in FIG. 1, when an upper-side switch UG is turned on and a lower-side switch LG is turned off, a current IL flows from the upper-side switch UG to an inductor L. As a result, the current IL flowing through the inductor L increases. On the other hand, when the upper-side switch UG is turned off and the lower-side switch LG is turned on, the current IL flows from the inductor L to the lower-side switch LG. As a result, the current IL flowing through the inductor L decreases. In addition, to prevent the upper-side switch UG and the lower-side switch LG from being turned on simultaneously, a dead time will be applied when controlling the turning on and turning off of the UG and LG.
As shown in FIG. 2A, when the current IL flowing through the inductor L is positive, during a dead time ΔT1 when the lower-side switch LG is turned off but the upper-side switch UG has not yet been turned on, and within a dead time ΔT2 when lower-side switch LG has not yet been turned on but the upper-side switch UG is turned off, the current IL flows from the inductor L and through a body diode of the lower-side switch LG. Therefore, a voltage at a node LX will be −0.7V if the turn-on voltage of the body diode of the lower-side switch LG is 0.7V. Also, the voltage at the node LX will be an input voltage VIN of the switching DC voltage converter when the lower-side switch LG is turned off and the upper-side switch UG is turned on. In this case, an equivalent on-time ton of the switching DC voltage converter is equal to an on-time of the upper-side switch UG.
As shown in FIG. 2B, when the current IL flowing through the inductor L is partially negative, within the dead time ΔT1 when the lower-side switch LG is turned off but the upper-side switch UG has not yet been turned on, the current IL flows from the inductor L and through the body diode of the upper-side switch UG since the current IL flowing through the inductor L is negative. Therefore, the voltage at the node LX will be the input voltage VIN of the switching DC voltage converter plus 0.7V if the turn-on voltage of the body diode of the lower-side switch LG is 0.7V. The on-time of the upper-side switch UG in FIG. 2A is equal to the on-time of the upper-side switch UG in FIG. 2B, but the equivalent on-time ton (i.e., the time duration when the voltage at the node LX is at high level) of the switching DC voltage converter in FIG. 2B is longer than the equivalent on-time ton of the switching DC voltage converter in FIG. 2A by a dead time ΔT1.
As shown in FIG. 2C, when the current IL flowing through the inductor L is entirely negative, within the dead time ΔT2 when lower-side switch LG has not yet been turned on but the upper-side switch UG is turned off, the current IL flows from the inductor L and through the body diode of the upper-side switch UG since the current IL flowing through the inductor L is negative. According to FIG. 2A and FIG. 2C, the on-time of the upper-side switch UG in FIG. 2A is equal to the on-time of the upper-side switch UG in FIG. 2C, but the equivalent on-time ton of the switching DC voltage converter in FIG. 2C is longer than the equivalent on-time ton (i.e., the time duration when the voltage at the node LX is at high level) of the switching DC voltage converter in FIG. 2A by a dead time ΔT1 and a dead time ΔT2.
According the above described operation of the switching DC voltage converter using a feedback control circuit with the constant on time mode circuit design, when the current IL flowing through the inductor L is partially or entirely negative, the equivalent on-time ton of the switching DC voltage converter is longer, which decreases the switching frequency of the switching DC voltage converter.